Broadly neutralizing anfibodies to HIV have the ability to prevent HIV infection in animal models and therefore it has been proposed that vaccines that elicit such antibodies might also be highly effective in humans. However, despite numerous efforts very few broadly neutralizing anfibodies have been identified to date. Several of these most potent broadly neutralizing antibodies are also polyreactive, i.e. they react with more than two other defined antigens, a property that has been attributed to long and hydrophobic or charged immunoglobulin heavy chain complementarity determining region 3 (CDR3). We have developed techniques to clone antibodies from single human B cells and studied thousands of antibodies obtained from B cells in defined stages of development. In these studies we showed that although polyreactivity is a normal feature of antibodies produced by nascent B cells in the bone marrow, this is normally selected against as B cells develop. It has been suggested that this selection is in part responsible for the rarity of broadly neutralizing anfibodies against HIV. The object ofthe proposed research is to use the human anfibody cloning technology we developed to determine whether B cells that produce broadly neutralizing anti-HIV anfibodies are selecfively deleted during B cell development and if so at which developmental stage. In addifion we propose to study B cells and antibodies from individuals that produce broadly neutralizing anfibodies to determine whether these patients show altered selection against polyreactive anfibodies. A third aim is to identify individuals with high titers of broadly neutralizing antibodies in their serum and to clone and characterize these anfibodies using our single cell methods. The long-term goals are to understand the genesis of anti-HIV anfibodies and to define additional anfigenic epitopes to be used in efficient vaccines developed by the Steinman and Ravetch laboratories to elicit protective anti-HIV antibodies